pgsql: Fix the tests added by commit ac4645c015.

Fix the tests added by commit ac4645c015.

In the tests, after disabling the subscription, we were not waiting for
the replication connection to drop from the publisher. So when the test
was trying to use the same slot to fetch the messages via SQL API, it
sometimes gives an error that the replication slot is active for other
PID.

Per buildfarm.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/266b5673b4b6bed2e9ebfe73ca967f44d6dc0e6c

Modified Files
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src/test/subscription/t/020_messages.pl | 8 ++++++++
1 file changed, 8 insertions(+)